6
Y. Ozaki and C. Huck
only with overtones and combination modes as a vibrational spectroscopy, it is very
much unique compared with Raman, IR, and Terahertz/FIR spectroscopy.
The NIR region can be divided into three regions; Region I (800–1200 nm; 12,500–
8500 cm
−1 ), Region II (1200–1800 nm; 8500–5500 cm
−1 ), and Region III (1800–
2500 nm; 5500–4000 cm
−1 ) [1–7]. The boarders of the three regions are not rigorous.
In Region I, bands arising from electronic transitions and those due to higher-order
overtones and various types combination modes are expected to appear. Region I
shows high transparency because all the bands appearing in this region are very
weak, allowing biomedical applications and applications to agricultural products.
Region I is the region where CCD cameras work very well, and this region is called
“window of biological materials” because of high transparency to body. It has also
a few more nick names: “the short-wave NIR (SWNIR) region,” ”near NIR (NNIR)
region,” or “the Herschel region.”
Region II is a region for the first and second overtones of the XH (X = C, O, N)
stretching vibrations and various types of combination modes. Region III contains
mainly bands attributed to the combination modes except for the second overtone of
the C = O stretching vibrational mode. It shows poorer “permeability.”
NIR spectroscopy, particularly vibrational NIR spectroscopy, is spectroscopy
of anharmonicity [1–7]. The overtones and combination modes are the so-called
forbidden transitions for a harmonic potential, yielding very weak bands. Both the
frequencies and intensities of NIR bands are controlled by anharmonicity. Therefore, investigations on overtones and combination modes, anharmonicity, vibrational
potential, and dipole moment function regarding NIR spectroscopy are important.
However, these studies have been far behind applications of NIR spectroscopy probably because until 1990s, it was difficult to obtain accurate NIR spectra and to make
reliable band assignments. It is only recent that quantum chemistry has been introduced to studies of frequencies and intensities of overtones and combination bands
(Chap. 5).
The fact that bands in the NIR region are weak or very weak is what makes this
region unique and markedly different from the other regions [1–7]. The reason why
the NIR region is valuable in various applications is because only the NIR region
serves as a highly transmitting window to radiation thanks to anharmonicity.
1.3 Brief History of NIR Spectroscopy
It is uncertain when NIR spectroscopy began, but there is the report that Abney and
Festing measured the spectra of some simple organic compounds in the 700–1200 nm
region as well as in the Vis and IR regions. In the beginning of the twentieth century,
main concerns of molecular spectroscopy were UV–Vis and IR spectroscopy. It was
1920s and 30s that systematic measurements of NIR spectra were carried out. A
chance came from the development of a spectrometer by Brackett. In 1930s, spectroscopists already recognized that NIR spectra arise from overtones and combination modes [9, 10]. In 1950s, NIR spectroscopy received considerable interest for
Y. Ozaki and C. Huck
only with overtones and combination modes as a vibrational spectroscopy, it is very
much unique compared with Raman, IR, and Terahertz/FIR spectroscopy.
The NIR region can be divided into three regions; Region I (800–1200 nm; 12,500–
8500 cm
−1 ), Region II (1200–1800 nm; 8500–5500 cm
−1 ), and Region III (1800–
2500 nm; 5500–4000 cm
−1 ) [1–7]. The boarders of the three regions are not rigorous.
In Region I, bands arising from electronic transitions and those due to higher-order
overtones and various types combination modes are expected to appear. Region I
shows high transparency because all the bands appearing in this region are very
weak, allowing biomedical applications and applications to agricultural products.
Region I is the region where CCD cameras work very well, and this region is called
“window of biological materials” because of high transparency to body. It has also
a few more nick names: “the short-wave NIR (SWNIR) region,” ”near NIR (NNIR)
region,” or “the Herschel region.”
Region II is a region for the first and second overtones of the XH (X = C, O, N)
stretching vibrations and various types of combination modes. Region III contains
mainly bands attributed to the combination modes except for the second overtone of
the C = O stretching vibrational mode. It shows poorer “permeability.”
NIR spectroscopy, particularly vibrational NIR spectroscopy, is spectroscopy
of anharmonicity [1–7]. The overtones and combination modes are the so-called
forbidden transitions for a harmonic potential, yielding very weak bands. Both the
frequencies and intensities of NIR bands are controlled by anharmonicity. Therefore, investigations on overtones and combination modes, anharmonicity, vibrational
potential, and dipole moment function regarding NIR spectroscopy are important.
However, these studies have been far behind applications of NIR spectroscopy probably because until 1990s, it was difficult to obtain accurate NIR spectra and to make
reliable band assignments. It is only recent that quantum chemistry has been introduced to studies of frequencies and intensities of overtones and combination bands
(Chap. 5).
The fact that bands in the NIR region are weak or very weak is what makes this
region unique and markedly different from the other regions [1–7]. The reason why
the NIR region is valuable in various applications is because only the NIR region
serves as a highly transmitting window to radiation thanks to anharmonicity.
1.3 Brief History of NIR Spectroscopy
It is uncertain when NIR spectroscopy began, but there is the report that Abney and
Festing measured the spectra of some simple organic compounds in the 700–1200 nm
region as well as in the Vis and IR regions. In the beginning of the twentieth century,
main concerns of molecular spectroscopy were UV–Vis and IR spectroscopy. It was
1920s and 30s that systematic measurements of NIR spectra were carried out. A
chance came from the development of a spectrometer by Brackett. In 1930s, spectroscopists already recognized that NIR spectra arise from overtones and combination modes [9, 10]. In 1950s, NIR spectroscopy received considerable interest for